Conventional methods of flame retardation for aromatic polycarbonate resins include addition of an organic halide compound, e.g., a polycarbonate oligomer of tetrabromobisphenol A as disclosed in U.S. Pat. No. 3,855,277, corresponding to Japanese Patent Publication No. 44537/72. Although this method is successful in shortening the combustion time, addition of such an organic halide compound alone cannot prevent dripping of resin melted upon combustion. Further, a rather large amount of a halide should be added for sufficiently shortening the combustion time, which leads to evolution of a large quantity of harmful gases or smoke upon combustion.
A series of Japanese patent applications including Japanese Patent Application (OPI) Nos. 54745/77 and 65555/77, etc. (the term "OPI" as herein used refers to "unexamined published application") disclose addition of organic sulfonic acid salts. The sulfonates recited in these patent applications are claimed to have a flame-retarding effect, especially an effect to inhibit dripping of molten resin during combustion, and, in addition, an effect to inhibit fuming. However, when an aromatic polycarbonate resin having incorporated therein such a sulfonate is molded at high temperatures, extensive decomposition of the polymer takes place, to not only cause poor appearance of molded products due to partial foaming, but also to reduce the mechanical strength of the molded products due to reduction of molecular weight. More specifically, a polycarbonate resin having incorporated therein a sulfonate having the formula: ##STR1## produces molded products of poor appearance due to partial foaming. Further, when a polycarbonate resin of 24,000 in molecular weight and a coloration value of 15 of APHA (American Public Health Association) having incorporated therein 0.3% by weight of a sulfonate having the formula: ##STR2## is molded, the molecular weight of the resin decreases to about 17,000 to 18,000 and coloration occurs due to remarkable thermal decomposition, resulting in a coloration value of 500 or more of APHA, as determined with respect to a solution of 4.0 g of the polycarbonate in 25 ml of methylene chloride.
Further, Japanese Patent Application (OPI) No. 50956/81 discloses addition of an organic phosphate of the formula ##STR3## wherein X represents a fluorine, chlorine, or bromine atom; m represents 0, 1, or 2; M represents a sodium, potassium, magnesium, calcium, strontium, barium, zinc, boron, or aluminum atom; and n represents an integer corresponding to the valency of M, and an oligomer of a halogenated bisphenol or a co-oligomer of a halogenated bisphenol and a non-halogenated bisphenol, to a polycarbonate resin. This method appears to be effective to some extent in inhibition of dripping of molten resin during combustion, suppression of reduction in molecular weight during molding or prevention of coloration during molding. However, it is still difficult for this method to stably meet the standard requirements of UL94V-0 of Underwriters' Laboratories, Inc., which is one of the severest standards of flammability required in the electric and electronic fields, and also it is naturally impossible to conform to the standard for flammability prescribed in Canadian Standards Association, Electrical Bulletin No. 968, Clause 6.11 (hereinafter referred to as "CSA flammability standard"), which is severer than UL94V-0.
According to the mechanism of flame-retardation of aromatic polycarbonate resins with alkali metal salts, it is known that the alkali metal salt catalyzes decomposition and rearrangement of the main chain bond of an aromatic polycarbonate resin under high temperature conditions, such as molding, to form a branched or crosslinked structure such as a salicylate bond, etc., to thereby improve a viscosity characteristics of a molten resin during combustion, as shown in the following reaction scheme: ##STR4##
However, in the case when the amount of the alkali metal salt is large, a branched or crosslinked structure is surely formed, but the decomposition of the main chain bond is excessively accelerated, making it difficult to prevent dripping of the molten resin and to obtain satisfactory molded products because of formation of silver streak, i.e., appearance irregularly distributed and often shaped like a comet locally, etc. If the amount of the alkali metal salt is limited so as to obtain satisfactory molded products, the formation of the branched or crosslinked structure becomes insufficient to improve the viscosity characteristics of the molten resin during combustion. The above-cited Japanese Patent (OPI) Applications are disadvantageous from the above-described viewpoints.